1. Field of the Invention
The present invention generally relates to a screw-type driving device which is constituted by a screw member, that is a screw shaft, and a nut member thread-engaged with the screw shaft, and in which the screw shaft or the nut member is rotated to thereby make them advance or retreat relative to each other, and particularly to a back-lash eliminating mechanism in such a screw-type driving device as described above, in which back-lash between the screw shaft and the nut member, which is difficult to be avoided in view of usage of thread-engagement, is eliminated.
2. Description of the Related Art
Conventionally, the screw-type driving device is arranged such that a screw shaft is rotatably supported by a fixed bracket and a nut member thread-engaged with the screw shaft is fixed to a movable bracket so as to be capable of making the movable bracket advance or retreat relatively to the fixed bracket.
When the screw shaft is rotated by rotary driving means such as a motor, or the like, the nut member is axially advanced or retreated on the screw shaft in accordance with the rotation thereof. Thus, the position of the nut member, that is the position of the movable bracket, relative to the fixed bracket is adjusted. Moreover, if rotation transmitting means is stopped, the nut member is fixed to the screw shaft because the former is thread-engaged with the latter. Further, there are two other kinds of screw-type driving devices, one being arranged such that a nut member is rotatably supported by a bracket and a screw shaft is fixed to another bracket, the other being arranged such that a nut member is made shaft-like and a screw shaft is made to be a mere screw member.
In the screw-type driving device of this kind, however, back-lash between the screw shaft and the nut member is unavoidable because of usage of a thread engagement. For this, the movable bracket may chatter relative to the fixed bracket. Accordingly, conventionally, there has been proposed a back-lash eliminating mechanism in which a ball screw was disposed in the thread-engagement portions between the screw shaft and the nut member in order to eliminate back-lash. Thus, the screw-type driving device were made larger in size because of its complex structure. Further, the device was expensive because of requirement of high machining accuracy.